Calibrating Reflectance Measurements to Predict Optimal Sidedress Nitrogen Rate for Corn

Optimal N fertilizer rate for corn (Zea mays L.) and other crops can vary substantially within and among fields. Current N management practices do not address this variability. Nitrogen-stressed corn reflects more visible light than N-sufficient corn, possibly providing a basis for diagnosing optimal sidedress N rate in a growing crop. Our objective was to calibrate reflectance measurements to predict economically optimal N rate (EONR) of corn in Missouri. We conducted eight field experiments with N rate treatments in which reflectance was measured at the V6 growth stage in plots that had received 0, 110, or ≥220 kg N ha ―1 at planting. Reflectance was measured in eight wavebands (510-830 nm) using a passive radiometer in three orientations (straight down, angled forward, and angled diagonally across rows). Reflectance ratios (relative to reflectance from high-N plots) were necessary to produce good relationships with EONR. We observed r 2 from 0.56 to 0.76 between relative reflectance and EONR for all visible (510―710 nm) wavelengths in all measurement orientations except 610 diagonal. The highest r 2 values were observed for relative 510-, 560-, and 710-nm reflectance in the straight down orientation. The straight down orientation is preferable for use in field applications because it gave predictions as good as or better than the other orientations, while avoiding the complexities associated with angled orientations. Combinations of wavelengths did not have a stronger relationship with EONR than individual visible wavelengths, but all visible/NIR ratios gave r 2 ≥ 0.43 in all orientations, with many above 0.6. Calibrations such as those presented here could be used to convert reflectance measurements to N rate recommendations during variable-rate sidedressing of corn.